2007 National Lake Survey - Wisconsin Results

Indicators of Lake Productivity

The trophic status of a lake refers to its overall level of productivity, or growth. A eutrophic lake has high nutrients and supports a large biomass (all the plants and animals living in the lake). They are usually either weedy or subject to algae blooms or both. In contrast, an oligotrophic lake has low nutrient concentrations and few plants. They are generally clear, deep, and free of weeds or large algae blooms. Though beautiful, they are low in nutrients and do not support large fish populations. Mesotrophic lakes fall somewhere in between with moderate nutrient inputs and plant growth. In Wisconsin we naturally find all three types of lakes.

To categorize lake productivity, the Survey measured the following indicators:

• Vertical profile measurements
• Secchi disc transparency
• Water chemistry: total phosphorus, dissolved silica, pH, alkalinity, total nitrogen, dissolved organic carbon, total organic carbon, total suspended solids , apparent color
• Chlorophyll-a

Vertical profile measurements

Depth profiles for temperature, pH, dissolved oxygen, and conductivity were taken from the deepest part of the lake. This information will be used to determine the extent of stratification, or separation of the water column into zones of differing temperature and density. It will also tell researchers whether the temperature and level of dissolved oxygen present is sufficient to support aquatic life.

Secchi disc transparency

As a measure of water clarity, a secchi disc measurement was taken from the middle of each Survey lake. A secchi disc is a weighted, 8-inch diameter, black and white painted disc that is lowered over the downwind, shaded side of the boat until it just disappears from sight. It's then raised until it is just visible and the average of the two depths is recorded.

Water chemistry

Water samples for total phosphorus, total nitrogen, basic anions and cations, dissolved organic carbon, alkalinity, total organic carbon, total suspended solids and conductivity were taken from the middle of each lake.

Total Phosphorus: Phosphorus is necessary for plant and animal growth and is often the limiting nutrient in aquatic systems. As a result, small increases in phosphorus in lake water can cause substantial increases in aquatic plants and algae. When phosphorus levels in lakes increase due to human activities (e.g. fertilizer, animal waste, soil erosion) eutrophication can occur.

Common total phosphorus values for Wisconsin:

Good: < 0.020 mg\L
Fair: 0.020 – 0.050 mg\L
Poor: 0.050 – 0.150 mg\L
Very Poor: 0.150 mg\L

Dissolved Silica: Dissolved silica is an important mineral for diatoms, singled-celled photosynthetic algae that occur in almost all fresh and saltwater bodies. Diatoms are highly nutritional and are an important base for most aquatic food webs. In systems where silica is extremely low it may become a limiting nutrient to diatom growth.

pH: pH is the inverse measure of the hydrogen ion concentration on a scale from 0 to 14, with 7 being neutral. Acidic lakes have a low pH with a high concentration of hydrogen ions, while basic lakes have a high pH with a low hydrogen ion concentration. Because pH controls many biochemical processes, it helps determine which plant and animals species can live in a lake. Typical lakes with a healthy fish population range in pH from 6.5 to 8.5. pH levels lower than 6.5 may be too acidic for many fish species, causing detrimental effects to fish spawning and heath.

Alkalinity: High levels of alkalinity are often associated with the ability of water to withstand becoming acidic from a stress like acid rain. These alkaline levels are a measure of carbonate or bicarbonate minerals suspended in the water. Hard water lakes, or those high in alkalinity, sometimes produce a solid carbonate precipitate called marl. These lakes also tend to produce more fish and aquatic plants than soft water lakes.

Common alkalinity values for Wisconsin:

Soft: 0-60 mg/L
Moderately hard: 61-120 mg/L
Hard: 121-180 mg/L
Very hard: >180 mg/L

Total Nitrogen: Nitrogen is second only to phosphorus in importance to aquatic plant and algal growth. Nitrogen concentrations can vary widely temporally and spatially but are often related to local land use. Humans can increase nitrogen in the watershed through waste treatment plants, fertilizers and runoff.

Common total nitrogen values for Wisconsin:

Oligotrophic: < 0.4 mg\L
Mesotrophic: 0.4-0.6 mg\L
Eutrophic: 0.6-1.5 mg\L
Hypereutrophic: 1.5 mg\L

Dissolved Organic Carbon: Dissolved organic carbon (DOC) is a measure of a wide range of plant and animal-derived organic compounds that have sufficiently broken down to become dissolved in lake water. Some DOC compounds affect lake water pH, while others stain the water a tea-like color. DOC is strongly influenced by the surrounding landscape of the water body.

Total Organic Carbon: Total organic carbon (TOC) is the amount of dissolved organic carbon (DOC) plus particulate organic carbon (POC) in an aquatic system. POC differs from its dissolved counterpart because the particles are large enough to become suspended in the water column but are not as readily taken up by microbial organisms.

Total Suspended Solids: Total suspended solids (TSS) is the amount of organic and inorganic particles suspended in the water column. TSS measures the weight of the particles and high values can have implications on light penetration, recreational value, and habitat value.

Apparent Color: Apparent color is a measure of naturally-occuring humic acids in lake water. Humic acids leach from vegetation in the watershed giving lake water the appearance of tea. Color is measured with Standard Platinum Units (SPU) and lakes greater than 25 SPU are considered 'colored'.

Chlorophyll-a

Chlorophyll-a is the pigment that makes plants and algae green. By measuring chlorophyll-a in lake water, you can indirectly measure the amount of photosynthesizing algae and phytoplankton in the water column.